An audio processing system such as a public address system or a hearing aid system compromises a microphone, an audio processing unit and a speaker (receiver in the case of a hearing aid). In the ideal audio processing system, the audio signal would flow in only a forward direction: from the audio source, to the microphone, to the audio processing unit, to the speaker (receiver), to the target eardrum.
In a non-ideal audio processing system, part of the acoustic audio signal generated by the speaker (receiver) returns back to the microphone. This phenomenon is called audio feedback, and the physical path that brings the receiver signal back to the microphone is usually known as an acoustic feedback path or leakage path.
The re-entry of the audio signal through the feedback path can cause artifacts that can vary from “voice in a pipe” effect, to ringing, to sustained oscillation (whistling or howling), which can cause discomfort to the listener, and may render the system unusable.
Oscillation due to feedback generates audible periodic signals, including audible tones, and audible signals with periodic components. At first glance, a simple periodic signal detector could be used to detect periodic feedback signals. However, there are several audio sources in the environment which generate tones and periodic signals, such as appliance alarms, phones and musical instruments, to name a few. Therefore, it is highly desirable to have a audio processing system that can make a distinction between an periodic environment signals and a legitimate periodic feedback signal such that the system can attenuate only legitimate feedback signals.